I am thinking about getting the 19inch stock c63 rims. I have 18s now. I was wondering how is the ride quality is with the 19s compared to the 18s?

With any 19 inch rims, the ride comfort will not be too much different... just a little harsher. But the tires will be thinner so watch for potholes as the rims will be more prone to bend.

The cornering is improved, due to less sidewall flex. The ride is harsher, and you feel every bump or imperfection is the rode more. The car is prone to bent wheels more, due to less sidewall. Avoid all potholes like the plague, or you'll be buying new wheels. Tires and wheels cost more. I have 19" on my E63, and it rides nice, but the E63 has an entirely different suspension, and I can soften up the ride. If you still choose to buy 19" wheels, get a good quality piece. Buying cheap wheels, will cost you more in the long run. Since the C63 has a pretty stiff already, it will be even worse. But handling, will be improved.

tad rougher, tad slower, tad heavier and 1 bent wheel

I agree with Chubbs...a tad rougher but you would probably not notice much, if at all. My wife drove mine today (I have the euro 19" wheels) and she thought the ride was smooth...now she wants my car!

A little rougher but not bad at all. I actually enjoy it better. Handling will be better if you go 265 in the rear if you plan on getting the euro 19s as most people have done.

thanks guys.

One benefit to going with 19" is it will look alot better. I personally would have like the other of 19" but I didn't order the car. The 19" option is only available if your ordering the car, not buying it off the lot. And I would definitely go with a wider rear tire 265. The choice is yours, another thing don't get rid of your 18" keep them to use during the winter months, mount on some winter tires, and you'll be good to go.

I notice a big difference in the ride being a bit more rough with my 19's